Thermal relay



April 1949- J. STOECKLIN ETAL THERMAL RELAY Filed Aug. 26, 1944 Patented Apr. 26, 1949 THERMAL RELAY Josef Stoecklin and Alfred Kach, Baden, Switzer-v land, assignors t Akticngcsellschaft Brown, Boverie & Cie., Baden, Switzerland, a jointstock company Application August 26, 1944, Serial No. 551,405 In Switzerland December 31, 1942 Section 1. Public Law 690, August 8, 1946 Patent expires December lg,- 1962 6 Claims. (01. 175-29 1) It is known that electric conductors, machines and apparatus can be protected against overloads by using thermal relays where the tripping element is built into a thermal image of the object which is to be protected. The term thermal image designates a device which has the same time-temperature characteristics as the electrical equipment to be protected when the device is heated by the current supplied to, or proportional to the current supplied to, the electrical equipment. This image is generally of the samekind as the protected object itself but reproduced to a smaller scale and in a simplified form.

,The prior thermal relays are open to the objections that some types are not readily adjustable for use with electrical equipment of diilerent constructions and sizes, and other types draw considerable power which imposes an undesirable load upon conventional current transformers. Furthermore it is often difficult to obtain a sufficiently large time constant, that is the time until the steady temperature is reached by the known thermal relay with thermal image, and thus attain a sufiiciently long interval of time until the tripping temperature is reached.

It must also be noted that the time-temperature' curve of electrical machines, apparatus and the like is not a uniform exponential curve, but during the heating resulting from increasing current, the slope of the time-temperature curve of the equipment passes from a higher to a lower value. This condition must also be satisfied by the thermal relay if it is to afford the intended protection in dependence upon the magnitude and previous mode of variation of the power current.

In the thermal relay of this invention, both the thermal tripping member and the heat-storing masses of the thermal image are heated in common by a resistance which is fed by a current proportional to the power current of the equipment to be protected, and the trippin member is combined into a block with the heat-storing masses and the heating resistance; heat insulation of the same or different heat transmission, as may be required, being arranged between the heating resistance and respectively, the tripping member and the heat-storage masses to obtain a displacement of the tripping member in accordance with the temperature of the electrical apparatus to be protected.

In the accompanying drawings:

Fig. 1 is a somewhat schematic view\ of a thermal relay embodying the invention, the

thermal image being shown in longitudinal section; and

Fig. 2 is a transverse section through the thermal relay as taken on line 2-2 of Fig. 1.

A current proportional to the power current of an electrical apparatus which is to be protected, for instance an electrical machine, is developed by a current transformer a and flows through a fiat strip or ribbon-shaped heating resistance b. The resistance strip b is a single inverted U-shaped loop in an open-ended duct 0, and this loop extends around the tripping member d which comprises a series of reversely bowed bimetallic plates which absorb a part of the heat developed by the heating resistance. The remainder of the developed heat is absorbed by the metal plate e; and the end wall of an open-ended metal frame 9 which form the boundary walls of the duct 0, the heating resistance I) being spaced from the duct walls by sheets of insulation f1. Sheets of insulation f2 are located between the heating resistance b and the tripping member d. Interchangeable heat storage plates ez of different mass are held in contact with the heated wall of the frame g by a spacer or filler plate u, and the block assembly of the heating resistance b and heat-absorbing members is encased within the metallic housing h which limits the escape of heat to the surrounding atmosphere. The block assembly is supported by bars 7' of low heat conductivity which are secured to a frame or base plate, not shown.

The total mass and the composition of the heat-storage plates 62 are selected in accordance with the time-temperature characteristic of the protected apparatus under various loadings. The inner metallic frame g is a fixed part of the thermal relay which, with the metal plate e1, has a constant heat-absorbing and heat-radiating characteristic that may be modified, as desired, by the introduction of heat-absorbing plates es of difierent mass.

The thermal tripping member 11 actuates over the axle i and the gear sector It a differential gearing l, m, 12 whose differential wheels I, m are provided with pointers. The pointer on the free wheel 1 indicates continuously on a scale the temperature of the protected object, whilst the wheel m and its pointer can be adjusted by hand to the desired tripping temperature. The thermometer pointer on the wheel I can be provided with a maximum indicator pointer not shown in the drawing for the purpose of indicatin the maximum temperature reached. The planet wheel 12 causes the tilting-lever o of switch p to turn and when the tripping temperature is reached this lever passes its dead centre and closes the release contact p. If the protected object becomes warm during no-load, for instance a transformer, then this no-load heating can be taken into account on the scale of the thermometer by malting the scale of the latter commence only at noload heating.

The lower support of the tripping member is connected with the bearing of the axle i by means of a rigid arm q, so that any displacement of the tripping member d within the conduit 0 has no effect on the temperature indication and tripping temperature.

The heating current in the resistance b can be adjusted by means of the variable shunt r to the desired value. It is also expedient to provide the usual compensation for the variable ambient temperature; this has not been shown in the drawing in order not to complicate the figure unnecessarily.

If it is desired to cause instantaneous tripping when large overload currents occur then an electro-magnetic tripping relay s is connected in series with the resistance b, this relay having two pairs of contacts one of which lies parallel to the tripping contact p of the thermal relay, whilst the other pair short-circuits the heating resistance b when the electro-magnet s is attracted. This limit-current release can of course also occur with a delayed action. The entire relay together with all the accessories is as usual enclosed in a casing t. Thermal and limit-release can be indicated by a common signal or by separate signals of the acoustical or optical kind either through a window in the casing t or at a remotely located spot.

As can be seen, the thermal image with the thermal relay according to the invention, is not at the same time a mechanical reproduction of the protected object. This possesses the advantage that the same relay can be used for different types of protected objects.

It is expedient to provide the relay with testing terminals which readily enable the relay to be tested in operation at the place of installation without undesirable switchin operations.

We claim:

1. A thermal relay for the protection of electrical apparatus of dififerent types and having a preselected time-temperature characteristic, when energized by a current proportional to the current supplied to a particular electrical apparatus, which is substantially identical with the time-temperature characteristic of said particular electrical apparatus; said thermal relay comprising thermal tripping means, heat storage means, heating resistance means in heat-transfer relation to both said thermal tripping means and said heat storage means, layers of heat insulation ping means and heat storage means to limit the radiation of heat therefrom; said heat storage means including a metal body constituting a minimum heat storage mass, and interchangeable metal plates in thermal contact with said metal body to increase the total heat storage mass to a magnitude which develops said predetermined time-temperature characteristic.

2. A thermal relay as recited in claim 1, wherein said metal body is an open-ended cylinder.

3. A thermal relay as recited in claim 1, wherein said metal body is an open-ended cylinder of square cross-section, said heating resistance means is adjacent one flat wall of said metal body, and said interchangeable metal plates contact the said flat wall of the metal body.

4. A thermal relay as recited in claim 1, wherein said heating resistance means includes parallel fiat strip resistors extending along opposite sides of and separated from said thermal tripping means by certain of said layers of heat insulation material.

5. A thermal relay as recited in claim 4, wherein said metal body of the heat storage means includes a fiat wall parallel to and spaced from one .of said fiat strip resistors by one oi said layers of heat insulation material.

6. A thermal relay as recited in claim 5, in combination with a metal heat-storage plate parallel to and spaced from the other fiat strip resistor by a layer of heat insulation material.

JOSE? S'IOECKLIN. ALFRED KAcH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,281,482 Baruch Oct. 15, 1918 1,308,048 Crichton July 1, 1919 1,429,660 Warner Sept. 19, 1922 1,477,455 Simmon Dec. 11, 1923 1,699,752 Randall Jan. 22, 1929 1,728,038 Burstyn Sept. 10, 1929 1,917,163 Stephens et al July 4, 1933 1,834,375 Bletz Dec. 1, 193i 2,056,040 Dosler Sept. 29, 1936 2,372,498 Keep Mar. 27, 1945 FOREIGN PATENTS Number Country Date 412,082 Great Britain June 21, 1933 

